Methods of corona location utilizing three-phase voltage distribution patterns

ABSTRACT

A method of locating a source of corona discharges in an electrical winding, comprising the steps of stressing the winding with a first voltage to provide a first voltage distribution pattern to ground and noting the magnitude of the first voltage at corona inception and corona extinction, and stressing the winding with a second voltage to provide a second voltage distribution pattern to ground, different than the first, and noting the magnitude of the second voltage at corona inception and corona extinction. The first and second voltage distribution patterns at the noted first and second voltages, respectively, for either corona inception or corona extinction, are then compared to determine where in the winding the voltages are identical on the two distribution curves.

United States Harrold atent- [72] Inventor: Ronald T. Harrold, Murrysville. Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

22 Filed: Dec. 9, 1970 21 AppLNo; 96,343

[52] US. Cl ..324/52, 324/54 [51] Int. Cl. ..G0lr 31/06, G0lr 31/08 [58] Field of Search ..324/51, 52, 54, 55

Ganger et al., Ionization Measurements on Transformers Offprint from the Brown Boveri Review 1967, No.7, pp. 3- 15.

[ 51 Aug. 15, 1972 Primary Examiner-Gerard R. Strecker AttorneyA. T. Stratton, F. E.. Browder and D. R. Lackey 57 ABSTRACT A method of locating a source of corona discharges in an electrical winding, comprising the steps of stressing the winding with a first voltage to provide a first voltage distribution pattern to ground and noting the magnitude of the first voltage at corona inception and corona extinction, and stressing the winding with a second voltage to provide a second voltage distribution pattern to ground, different than the first, and noting the magnitude of the second voltage at corona inception and corona extinction. The first and second voltage distribution patterns at the noted first and second voltages, respectively, for either corona inception or corona extinction, are then compared to deter mine where in the winding the voltages are identical on the two distribution curves.

7 Claims, 18 Drawing Figures CORONA INDICATING MEANS PATENTEDAUB 15 m2 SHEET 1 0F 5 CORONA INDICATING MEANS INVENTOR Ronald T. Horrold WITNESSES 0 /6E/ ATTORNEY PATENTEUAUG 15 I972 SHEET 2 OF 5 I r 50 6O 64 NUMBER OF DISCS OR WINDING SECTIONS PAIEN'IEHAUGIS I972 3.684.951

sum 3 BF 5 IGIkv PHASE VOLTAGE-SECONDARY 293W VOLTAGE AT EACH END OF WINDING TO GROUND (EI93kv FIGG.

VOLTAGE AT CENTER OF WINDING TO GROUND 46.5kv

EFFECTIVE 4 GROUND I6I kv PHASE VOLTAGE- SECONDARY FIG.7.

EFFECTIVE GROUND FOR SECONDARY AND SOLID GROUND FOR PRIMARY PATENTEDAUE 1 5 m2 3.684.951

SHEET u 0F 5 CORONA INDICATING "6 MEANS CORONA lNDlCATlNG MEANS HG FIG. 8C.

IOO

CORONA INDICATING MEANS METHODS OF CORONA LOCATION UTILIZING THREE-PHASE VOLTAGE DIST UTION PATTERNS BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates in general to testing electrical inductive apparatus, such as electrical power transformers, and more specifically to locating sources of corona discharges within such apparatus.

2. Description of the Prior Art Certain types of high voltage electrical apparatus, such as liquid filled power transformers, are tested after manufacturing and prior to shipment to non-destructively overstress the electrical insulation between the phases, between the conductor turns of the phases, and from the phase windings to ground, to a predetermined factor of safety. If the electrical apparatus passes these tests, it assures that the insulation will perform as expected at the intended operating voltage of the apparatus.

If corona or partial discharges are detected during these tests, it is important to measure the amount of energy in the discharges and to locate their source.

Partial discharges are a rapid discharge of electrical energy which ionizes, deteriorates, and may eventually break down the surrounding insulation through the heat produced by the electrical discharge, and resulting chemical changes in the organic insulation. There are many locations in high voltage electrical apparatus, such as power transformers, where a corona discharge may initiate, some of which may cause rapid deterioration of the surrounding insulation, and others which may cause little or no damage, even over a long period of time. Thus, it is important to test high voltage electrical inductive apparatus for the presence of corona discharges, during the normal test procedures for such apparatus, but it is not sufficient to simply determine if corona discharges are present in the apparatus. The specific location of the corona discharge source in the inductive apparatus should also be determined, to enable design engineering personnel to evaluate the type and severity of the partial discharge, to determine the corrections which may be necessary in the apparatus under test, and also to provide information useful in developing future insulating structures.

If it is necessary to drain the oil from the electrical apparatus, remove the winding from the tank, and unstack the coils, in order to repair the defect causing the corona, it is very important to have the corona source pinpointed before disassembly of the apparatus, as it is usually difficult to visually observe the corona site after disassembly of the apparatus. Therefore, accurate methods of locating the site or source of corona discharges are continually sought.

The methods conventionally used to locate corona within electrical windings are the sonic methods and the electrical methods. In the sonic methods, mechanical to electrical transducers are placed on the sidewalls of the electrical apparatus, and they detect mechanical disturbances within the apparatus resulting from corona discharges. The location of the corona source is narrowed to a specific location within the apparatus by triangulation. In the electrical methods, the corona is measured at each end of the winding, and corona located by the amount of attenuation of the corona discharge signals as measured at the winding ends. In other words, if the signals are of equal magnitude at each end of the winding, it is assumed that the corona source is at the midpoint of the winding.

With both the sonic and the electrical methods, sensitivity is a problem. For example, the sonic methods are useful for locating corona sources having magnitudes in the range of about 500 to 1,000 p. V, and up, and the electrical methods for sources greater than I00 p. V. Further, the electrical methods require measurements at opposite terminals of a winding, and often the signal will be measurable at only one terminal of the winding, which then rules out the use of the electrical method.

It would be desirable to be able to locate sources of corona discharges having energy contents down to the corona free level, defined as less than 10 pC, which for most power transformers will be approximately 5 p. V, the ambient reading for most radio noise meters used in corona testing.

SUMMARY OF THE INVENTION Briefly, the present invention is new and improved methods of locating the site of corona discharges in electrical windings, which method is sensitive enough to locate any corona site that can be detected. Depending upon the type of corona source, either the corona inception voltage at the corona site, or the corona extinction voltage at the corona site, with respect to ground, will be repeatable on successive tests. The invention uses at least two winding tests, which tests are selected to stress the winding with different voltage distribution patterns with respect to ground. The voltage distribution patterns for each test, using the test voltage magnitudes at which corona inception, or extinction, was detected, are then examined to determine where in the winding the two different patterns have the same voltage to ground, which pinpoints the corona source. The two different voltage distribution patterns may be obtained by the applied potential and induced potential tests, which are required by the USAS transformer standards for delta connected transformer windings, or by the proposed new induced tests for transformers in which different phases of a delta connected winding are grounded in sequence. Thus, the disclosed methods may utilize the results of the tests currently conducted on high voltage electrical inductive apparatus, making it unnecessary to add additional tests, or testing apparatus and procedures, and making it unnecessary to provide special training for test personnel. Further, the tests do not require a shielded room, and it is not necessary to make absolute measurements of the amount of energy in the corona discharges, as it is only necessary to accurately measure the winding test voltage when corona inception and corona extinction is noted.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understood and further advantages and uses thereof more readily apparent, when considered in viewof the following detailed description of exemplary embodiments, taken with the accompanying drawings, in which:

FIG. 1 is a partially schematic and partially block diagram, illustrating an applied potential test on an electrical power transformer, which test is a step in a method of locating a corona source according to a first embodiment of the invention;

FIG. 2 is a partially schematic and partially block diagram, illustrating an induced potential test on the electrical power transformer shown in FIG. 1, which test is another step in the method of locating a corona source according to the the first embodiment of the invention;

FIGS. 3 and 4 are vector diagrams of the electrical power transformer shown in FIG. 2, for the induced potential test;

FIG. 5 is a graph illustrating the phase voltage distribution with respect to ground for the induced potential test shown in FIG. 2 for the test voltage magnitude at which corona extinction was observed;

FIG. 6 is a vector diagram illustrating the location of a corona source external to the high voltage winding of the transformer shown in FIGS. 1 and 2;

FIG. 7 is a vector diagram illustrating the location of a corona source between the high and low voltage windings of the transformer shown in FIGS. 1 and 2;

FIGS. 8A, 8B and 8C are partially schematic and partially block diagrams illustrating the steps of another method of locating a corona source, according to the teachings of the invention, using only induced potential tests;

FIGS. 9A, 9B and 9C are graphs which illustrate the voltage distribution to ground for each phase during the test shown in FIG. 8A, at the winding voltage at which corona extinction occurred;

FIGS. 10A, 10B and 10C are graphs which illustrate the voltage distribution to ground for each phase during the test shown in FIG. 8C at the test voltage at which corona extinction occurred;

FIG. 11 is a graph which compares the two different voltage distributions across one of the phase windings shown in FIG. 8A, taken from the graphs in FIGS. 9A and 10A, illustrating that the corona source is not in that specific phase; and

FIG. 12 is a graph comparing the two different voltage distributions across another of the phases, taken from the graphs shown in FIGS. 9C and 10C, and illustrating that the corona source is in this phase, and the specific location of the corona source within the phase winding.

DESCRIPTION OF PREFERRED EMBODIMENTS The present invention discloses new and improved methods of locating corona sources, which methods may utilize the presently used applied potential and induced potential tests, or the proposed induced potential tests in which different winding terminals of a delta connected winding are successively grounded.

The new and improved methods are based on the realization that eitherthe corona inception voltage at the corona site, or the corona extinction voltage at the corona site, will be identical from test to test, with certain types of corona sources producing corona extinction voltages at the site which are of like magnitude from test to test, and other types of corona sources providing corona inception voltages at the corona site of like magnitude from test to test. For example, corona producing voids located between pressboard barriers in oil, produce corona extinction voltages, at the corona site, which are consistently the same from test to test.

If the winding test voltage at which corona extinction is noted is not consistently repeatable from test to test, using the same type of test, the corona inception values may be used, and when the corona inception values of the test voltage are not substantially similar from test to test, using similar tests, accurate results have been obtained by taking the average of the noted test voltages at which corona inception occurred for four or five similar tests.

Broadly, the invention utilizes the repeatability of the corona extinction or inception voltages, at the corona site, along with the steps of stressing the winding to be tested to provide two different voltage distribution patterns to ground across the winding, with the distribution patterns being developed by using the specific test voltage for each distribution pattern at which corona inception was noted, or corona extinction, as hereinbe fore stated. Then, the two voltage distribution patterns are compared to determine where in the winding the voltage is of like magnitude in the two patterns, which will accurately pinpoint the corona source. The comparison of the voltage distribution patterns may be accomplished graphically, or vectorially, as desired.

The new and improved methods of locating corona sources may be more easily understood by considering specific examples, with a first embodiment of the invention developing the required information for locating corona sources according to the teachings of the invention, by using the standard applied potential and induced potential tests, now used to test delta connected power transformers. The applied and induced potential tests produce different voltage distribution patterns across the winding tested, with respect to ground, and thus these patterns may be used to provide the at least two different voltage distribution patterns required by the method.

Referring now to the drawings, and FIG. 1 in particular, there is illustrated a partially schematic and partially block diagram of a step in a first embodiment of the invention, illustrating the applied potential test, which uniformly stresses the insulation of the three phases of a power transformer 10 to ground. Power transformer 10 includes a tank or casing 12 containing three-phase high and low voltage windings l4 and 16, respectively, which are disposed in inductive relation with a magnetic core (not shown) and immersed in a suitable fluid insulating and cooling dielectric, such as mineral oil, askarel, or SF gas. The low voltage winding 16 includes phase windings 18, 20 and 22, connected in wye, in this example, with the outwardly extending ends of the phase windings 18, 20 and 22 being connected to the encased ends of low voltage bushings 24, 26 and 28, respectively, and the common or neutral terminal of the wye connection is grounded at 30. The high voltage winding 14 includes phase windings 32, 34 and 36, connected in delta, in this example, with the line terminal 39, to which phase windings 32 and 34 are connected, being connected to the encased end of high voltage bushing 38, line terminal 37, to which phase windings 34 and 36 are connected, is connected to the encased end of high voltage bushing 40, and line terminal 41, to which phase windings 32 and 36 are connected, is connected to the encased end of high voltage bushing 42. High voltage bushings 38, 40 and 42 commonly have bushing taps 46, 48 and 50, respectively which are capacitively coupled to the main conductor studs of the high voltage bushings, and corona discharges may be conveniently detected at these bushing taps.

While the first embodiment of the invention is most applicable to delta connected high voltage windings, it is to be understood that it may also be applied to any type of winding, as long as care is taken to insure that the voltages developed within the winding during test do not exceed the maximum allowable voltage for all parts thereof.

In the applied potential test, shown in FIG. 1, a source 52 of low frequency alternating potential, commonly 60 or 180 hz., is applied to the winding terminals 38, 40 and 42 which voltage stresses all of the phases uniformly to ground. The source voltage 52 is increased in magnitude, toward a predetermined maximum test voltage, which depends upon the BIL voltage rating of the apparatus. The winding 14 is monitored for partial discharges or corona, during this test. While the method may be used with any means which indicates corona inception and extinction, it is preferable to use the most sensitive corona detecting means available, such as a radio noise meter; which reads the radio frequency energy detected in p.V, or a picocoulomb meter, which measures electrical charge per cycle. A pulse counter may be used in conjunction with these devices, if desired.

The corona may be detected by connecting the corona detecting means, indicated by reference numeral 54 in FIG. 1, to the bushing tap of any of the high voltage bushings 38, 40 and 42, such as indicated by the solid line 56 between bushing tap 46 and corona indicating means 54. Suitable isolating chokes, for shorting line frequency voltage from the bushing tap to ground, and a tuning capacitor connected from the bushing tap to ground, may be used in conjunction with the corona indicating means, in a manner well known in the art. If the bushing taps are not provided on the high voltage bushings, any other suitable means for coupling the corona detecting means 54 to the winding may be used, such as by using an auxiliary capacitive pickup with the NEMA test circuit.

If corona discharges are detected as the magnitude of ing means. When corona discharges are detected by corona detecting means 54, before reaching the maximum test voltage, the phase voltage of the high voltage winding 14 at which corona starts is noted. The source voltage is then gradually reduced until corona extinctionoccurs, and the phase voltage of the high voltage winding 14 is again noted. The corona readings at the three bushing taps will usually indicate by their relative magnitudes, in microvolts (uV), or picocoulombs (pC), in which phase the corona source is located, and often in which half of the phase it is located.

In the induced potential test, the turn-to-tum insulation throughout each phase is equally stressed, but the voltage to ground will vary throughout each phase winding, from a maximum at the ends of the windings to a minimum at the winding center. The exact voltage to ground at different positions along each phase may be determined by considering an effective ground for the high voltage winding 14, for the transformer as energized under no-load conditions and with the windings have substantially equal ground capacitance. If the windings do not have equal capacitance to ground, the ground, for purposes of a vector diagram, may be found by measuring the voltage to ground at two terminals.

FIG. 3 is a vector diagram of the transformer 10 shown in FIGS. 1 and 2, with vectors L1, L2, and L3 illustrating the primary or low voltage phase windings 18, 20 and 22, respectively, and vectors H1, H2 and H3 illustrating the secondary or high voltage phase source 52 is gradually increased towards the maximum test voltage, the magnitude of source 52 at corona inception is noted, and the reading of the corona indicating means, such as .1. V or picocoulombs, is noted. The magnitude of source voltage 52 is then gradually reduced until corona extinction occurs, with the magnitude of source 52 at corona extinction being noted.

The next step of the new and improved method of corona location is shown in FIG. 2, which provides a different voltage distribution pattern across the phase windings of the transformer 10, with respect to ground. This different voltage distribution pattern is obtained by using the induced potential test, in which a threephase low frequency source 62 of alternating potential is applied to the outwardly extending end of the low voltage bushings 24, 26 and 28, and the stressing voltage for the high voltage winding 14 is induced therein. The source voltage is gradually increased towards a predetermined maximum test voltage, and the bushing taps are monitored for corona discharges at each voltage step, either by switching from tap to tap with a single indicating means, or by using three separate indicatwindings 34, 36 and 32, respectively. The effective ground for winding 14 is indicated at 64. Thus, the high voltage and low voltage vectors, with respect to ground, for transformer 10, are as shown in FIG. 4.

From the vector diagrams shown in FIGS. 3 and 4, the actual phase voltage distribution, with respect to ground, for one high voltage phase winding and one low voltage phase winding, may be determined, for any phase voltage at which corona extinction or inception occurs. For example, the maximum voltage to ground of a selected phase, such as vector H2, would be equal to one-half of vector H3 divided by sine 60, and the minimum voltage, indicated by dotted line 66, would be equal to one-half of vector H3 divided by tan 60. As an example, the phase voltage distribution with respect to ground, during an induced test of a 1 l5 kv delta connected winding having 64 discs or winding sections per phase, in which 185 kv is the maximum test voltage for both the applied and induced tests, the insulation would be uniformly stressed at 185 kv to ground for the applied test, while for the induced test the maximum voltage to ground would be 107 kv i.e., 185 divided by 2 sine 60, at the ends of the winding, and 53.5kv i.e., 185 divided by 2 tan 60, at the center of the winding, as shown in FIG. 5. FIG. 5 is a graph which plots kilovolts to ground versus the number of discs or winding sections for the winding of this example, with curve illustrating the voltage to ground across the high voltage winding during the induced test. Curve 72 illustrates the voltage to ground for the low voltage phase winding, which is zero at its grounded end and 13.2 kv at its other end.

Since the voltage to ground at which corona inception or extinction occurs in the applied test is the magnitude of the applied voltage at that time, and since the voltage to ground pattern may be determined for a phase winding during the induced test, at the corona inception or corona extinction voltage, the location on the induced voltage pattern which is the same as the noted applied voltage, will locate the corona source.

For example, assume that with the applied test corona extinction consistently occurred at a winding voltage of 69 kv, that corona extinction occurred at 161 kv phase voltage during the induced potential test, and that from the p.V or picocoulomb readings at the bushing taps it was determined that the corona source is in phase H3, and in the half of phase H3 which is directly connected to phase H2. Because the corona was consistently extinguished at 69 kv during the applied test, it can safely be assumed that the corona was extinguished at 69 kv during the induced test. Thus, it is necessary to determine where in the winding a stress of 69 kv occurs when the phase voltage is 161 kv during the induced test. One location for the stress will be outside the high voltage winding, and another location, if the transformer has concentrically adjacent high and low voltage windings, will be between the high voltage and low voltage windings. These locations may be determined graphically, or vectorially, as desired.

The vectors for a corona source external to the high voltage winding are shown in FIG. 6. The minimum voltage is 46.5 kv for a-phase voltage of 161 kv, and the maximum voltage is 93 kv to ground. The angle 1 and the vector y at which corona extinction occurs is 69 kv. The angle 1 may be found by cosine I 45.5/69, or about 48. The angle is thus about 12 degrees and the voltage vector b from the corona source to the end of the winding is equal to 2y sine 6 or 29.8 kv. Since there are 64 discs in the winding, the location of the corona source may be found by multiplying the ratio of 29.8/161 times the number of discs 64, which locates the corona approximately 12 discs from end 37 of the phase winding H3.

To determine where the 69 kv stress occurs between concentrically adjacent high and low voltage windings, the vector diagram shown in FIG. 7 is used, which illustrates that the primary voltage to ground (9 kv in this example) is vectored with the 69 kv, to provide a voltage of 22.6 kv from the end 37 of the phase winding, which when multiplying the ratio of 22.6/ 161 by the number of discs 64 indicates that the corona source is in the ninth disc from the end 37 of the phase winding. Thus, the search for the corona source has been narrowed to the location between the ninth and twelfth discs from end 37 of the phase winding.

The method of noting the test voltage applied to a winding at corona inception and corona extinction, on successive tests, and using either the inception or extinction voltages to find the corona source, may also be used with the winding voltage distributions, rather than voltage vectors. This approach will become especially important if the USAS transformer standards are changed. As hereinbefore stated, the present USAS standards require that a delta connected transformer must be low frequency tested by means of both applied and induced potentials. Since the applied test stresses all parts of the winding to the full test voltage, and since in practice under the worst fault condition the voltage at the midpoint of the winding can reach only about 80 percent of the voltage at the line end, the winding must be insulated to withstand a test which will never be duplicated in operating practice. Thus, a new test procedure is under consideration which would allow insulation added to the midpoint of the winding, merely to pass the applied test, to be eliminated. This proposed new test is an induced test, which duplicates the worst possible operating condition during the test by successively grounding the three line terminals of a delta connected high voltage winding.

FIGS. 8A, 8B and 8C are schematic diagrams which illustrate the proposed new testing method, with FIG. 8A illustrating a transformer 80 having a primary or low voltage winding 82, and a secondary or high voltage winding 84. Low voltage winding 82 has phase windings 86, 88 and 90 connected in wye, with the outwardly extending ends 92, 94 and 96 of phase windings 86, 88 and 90 being connected to a source 100 of alternating potential, such as 60 or 180 hz., and their commonly connected ends are grounded at 98.

High voltage winding 84 has phase windings 102, 104

' and 106 connected in delta, having line terminals 108,

110 and 112. The high voltage winding 84 is tested by successively grounding the line terminals 108, 112 and 1 10 as illustrated in FIGS. 8A, 8B and 8C, respectively, increasing the induced voltage in winding 84 to the maximum test voltage.

In this embodiment of the invention, the bushing taps associated with the ungrounded terminals are monitored by corona indicating means 116, Le, such as a radio noise meter or a picocoulomb meter, on two successive tests, indicated by solid line 118 and dotted line 120, or simultaneously by using two similar corona indicating means. The source voltage is gradually increased, and if corona is detected, the phase voltage of winding 84 is noted, and then the voltage is gradually reduced until corona extinction occurs, at which time the phase voltage of winding 84 is again noted. The voltage distribution to ground, across each phase winding, at the corona inception or corona extinction voltage, whichever is selected, as hereinbefore explained relative to the first embodiment of the invention, is

plotted on a graph for each test in which corona discharges are detected, for the three tests shown in FIGS. 8A, 8B and 8C, and then the voltage distribution curves for like phases compared. The intersection of two curves for a phase indicates the location of a corona site within the phase.

For purpose of example, assume that a corona site is located within phase winding 102 at a site which is 20 percent of the winding length from line terminal 112. The first test, shown in FIG. 8A, will detect corona discharges at the bushing tap for terminal 112, and it will be assumed that the phase voltage to ground at corona extinction was measured at 100 kv. Since corona discharges were detected during this first test, shown in FIG. 8A, the voltage distribution across each phase of winding 84 is plotted, using the 100 kv corona extinction voltage as the terminal voltages of the winding, as illustrated in FIGS. 9A, 9B and 9C. FIG. 9A plots the voltage to ground across the length of phase winding 106, which will be 100 kv at each end of the winding and 87 kv at the center of the winding. FIG. 9B plots the voltage across phase winding 104, which is ground or zero voltage at one end and 100 kv at the other. FIG. 9C plots the voltage across phase winding 102, which is ground or zero voltage at one end and 100 kv at the other end.

The ground 114 is then removed from terminal 108, a ground 114 is applied to terminal 112, and the second test, shown in FIG. 8B, is performed, while detecting corona at the bushing taps for terminals 108 and 110. In this test, no corona discharges are detected at either terminal, even at the maximum test voltage of I85 kv to ground, since the corona source is near terminal 112, which is grounded, and the voltage at the corona site thus never reaches the corona threshold level. Since no corona discharges are detected in test 2, it will not be necessary to plot voltage distribution patterns for test 2, and it should now be apparent that the corona source is closer to terminal 112 than it is to terminals 108 or 110, and that the site must be located in phase winding 102 or in phase winding 106.

The ground 114' is then removed from terminal 112, a ground 114" is applied to terminal 110, and the third test, shown in FIG. 8C is performed while detecting corona discharges at the bushing taps for terminals 108 and 112. During this test, corona is detected at the bushing taps for both terminals 108 and 112, and corona extinction was measured at a terminal voltage of 88 kv to ground. Since corona discharges were detected during this third test, the phase voltage distribution to ground was plotted for each phase, with FIGS. 10A, 10B and 10C being graphs which illustrate the voltage magnitudes to ground across phase windings 106, 104 and 102, respectively. FIG. 10A plots the voltage to ground across the length of phase winding 106, which is ground at one end and 88 kv at the other end, FIG. 10B plots the voltage to ground across phase 104, which is also ground at one end and 88 kv at the other and FIG. 10C plots the voltage to ground across phase winding 102, which is 88 kv at each end of the winding and 76.5 kv at the center thereof.

The next steps are to plot the voltage distribution pattern to ground for phase 106, during each of the tests in which corona was detected, with the curves being plotted on the same graph, and also to plot the voltage distribution to ground for phase 102 for each of the tests in which corona was detected, with these curves being plotted on the same graph. The voltage distribution patterns are only plotted for these phases, since it was determined that they are the only phases in which the corona site could be located.

FIG. 11 is a graph which combines the two voltage distribution curves shown in FIGS. 9A and 10A, and since the two curves do not intersect, it is obvious that the corona site is not in phase 106.

FIG. 12 is a graph which combines the two voltage distribution curves shown in FIGS. 9C and 10C. These curves intersect at point P, and since the extinction voltage at the corona site is identical on successive tests, the corona location can only be at the intersection of these two voltage distribution curves. Therefore, from the three tests performed, illustrated in FIGS. 8A, 8B and 8C, it is definitely established that a corona site is located in phase 102, percent of the winding length from terminal 1 12.

In summary, there has been disclosed new and improved corona locating methods for electrical windings which locate corona by developing at least two different voltage distribution patterns to ground for the winding under test, with the patterns being established for the winding voltage at corona inception, of corona extinction, and with the corona being located by determining where identical voltages exist in the two voltage distribution patterns. The disclosed methods are extremely sensitive, locating any corona source which can be detected. The methods do not require a shielded room or special tests, as the standardized tests for high voltage electrical inductive apparatus provide all of the information required to locate corona sites, according to the teachings of the invention. Absolute measurements of corona discharge energy are not required. All that is necessary is to measure the test voltage at which corona inception and corona extinction occurs. Relative values of the corona energy will indicate which phase, and which half of the phase, the corona site is located. Still further, while the methods have been described relative to locating a single corona source, it will be apparent that more than one source of corona discharge may be located, if they have different inception or extinction voltages. While the two embodiments of the invention have been described as alternative methods of locating a corona source, they may both be used on a single transformer, if desired, if more than two voltage distribution patterns are desired to increase the accuracy of the method.

I claim as my invention:

1. A method of locating a source of corona discharges in a three-phase delta connected electrical winding having first, second and third terminals, comprising the steps of:

stressing the three-phase winding with an applied voltage to provide a first voltage distribution pattern in which the winding is stressed uniformly to ground,

changing the magnitude of the applied voltage in a predetermined direction,

detecting corona discharges in the winding, noting the phase in which they occur, and noting the magnitude of the applied voltage when the corona condition of the winding changes,

stressing the three-phase winding with an induced voltage to provide a second voltage distribution pattern, different than the first voltage distribution pattern, which equally and uniformly stresses the phases of the winding turn-to-tum,

changing the magnitude of the induced voltage in the same direction as the applied voltage,

detecting corona discharges in the winding and noting the magnitude of the induced voltage when the corona condition of the winding changes,

and comparing the first and second voltage distribution patterns at the noted magnitudes of the applied and induced voltages, respectively, determining where in the noted phase winding the voltages are of like magnitude in the first and second voltage distribution patterns.

2. The method of claim 1 wherein the first and second voltages are changed to increase their magnitude, an the corona condition of the winding at which the magnitudes of the first and second voltages are noted is the inception of corona in the windings.

3. The method of claim 1 wherein the first and second voltages are changed to decrease their magnitude, and the corona condition of the winding at which the magnitudes of the first and second voltages are noted is the magnitudes at corona extinction.

4. The method of claim 1 wherein the steps are repeated a plurality of times and the magnitude of the first and second voltages at inception are averaged, to set the first and second voltage distribution patterns which are compared to locate the source of corona.

5. A method of locating a source of corona discharges in a three-phase delta connected winding having first, second and third terminals, comprising the steps of successively testing the winding with a different terminal grounded during each test, with each test including the steps of inducing a voltage into the winding, changing the voltage magnitude in a predetermined direction, and noting the magnitude of the winding voltage at which the corona condition of the winding changes, locating the phase in which the corona discharge is occurring and the location of the discharge within the phase by comparing the voltage distribution pattern of each phase when a terminal of the selected phase is grounded, with the voltage distribution pattern of the phase when its terminals are not grounded,

and determining in which phase, and where in that phase, the voltages of the distribution patterns are of like magnitude.

6. The method of claim 5 wherein the first and second voltages are changed to increase their magnitude, and the corona condition of the winding at which the magnitudes of the first and second voltages are noted is the voltages at corona inception.

7. The method of claim 5 wherein the first and second voltages are changed to decrease their magnitude, and the corona condition of the winding at which the magnitudes of the first and second voltage are noted is corona extinction. 

1. A method of locating a source of corona discharges in a three-phase delta connected electrical winding having first, second and third terminals, comprising the steps of: stressing the three-phase winding with an applied voltage to provide a first voltage distribution pattern in which the winding is stressed uniformly to ground, changing the magnitude of the applied voltage in a predetermined direction, detecting corona discharges in the winding, noting the phase in which they occur, and noting the magnitude of the applied voltage when the corona condition of the winding changes, stressing the three-phase winding with an induced voltage to provide a second voltage distribution pattern, different than the first voltage distribution pattern, which equally and uniformly stresses the phases of the winding turn-to-turn, changing the magnitude of the induced voltage in the same direction as the applied voltage, detecting corona discharges in the winding and noting the magnitude of the induced voltage when the corona condition of the winding changes, and comparing the first and second voltage distribution patterns at the noted magnitudes of the applied and induced voltages, respectively, determining where in the noted phase winding the voltages are of like magnitude in the first and second voltage distribution patterns.
 2. The method of claim 1 wherein the first and second voltages are changed to increase their magnitude, an the corona condition of the winding at which the magnitudes of the first and second voltages are noted is the inception of corona in the windings.
 3. The method of claim 1 wherein the first and second voltages are changed to decrease their magnitude, and the corona condition of the winding at which the magnitudes of the first and second voltages are noted is the magnitudes at corona extinction.
 4. The method of claim 1 wherein the steps are repeated a plurality of times and the magnitude of the first and second voltages at inception are averaged, to set the first and second voltage distribution patterns which are compared to locate the source of corona.
 5. A method of locating a source of corona discharges in a three-phase delta connected winding having first, second and third terminals, comprising the steps of successively testing the winding with a different terminal grounded during each test, with each test including the steps of inducing a voltage into the winding, changing the voltage Magnitude in a predetermined direction, and noting the magnitude of the winding voltage at which the corona condition of the winding changes, locating the phase in which the corona discharge is occurring and the location of the discharge within the phase by comparing the voltage distribution pattern of each phase when a terminal of the selected phase is grounded, with the voltage distribution pattern of the phase when its terminals are not grounded, and determining in which phase, and where in that phase, the voltages of the distribution patterns are of like magnitude.
 6. The method of claim 5 wherein the first and second voltages are changed to increase their magnitude, and the corona condition of the winding at which the magnitudes of the first and second voltages are noted is the voltages at corona inception.
 7. The method of claim 5 wherein the first and second voltages are changed to decrease their magnitude, and the corona condition of the winding at which the magnitudes of the first and second voltage are noted is corona extinction. 